Many off road work vehicles, such as a four wheel drive loader of backhoe, can include a work implement such as a bucket or boom. Referring to FIG. 1, an exemplary embodiment is shown of an off road work vehicle in the form of a four wheel drive loader 100. The vehicle 100 includes a front frame assembly 102 and a rear frame assembly 104 that are pivotally joined together at an articulation pivot or joint 116. The front frame assembly 102 can be supported by a front drive wheel 110 and the rear frame assembly 104 can be supported by a rear drive wheel 112. The front frame assembly 102 is also provided with a work implement in the form of a loader bucket 106 that is controllably coupled to the front frame assembly 102 by a coupler or mechanical linkage 108. In other embodiments, the front frame assembly 102 can be coupled with a pair of forks, a blade, a rotary tiller, a roller level, a rotary cutter, a trencher, and other known work implements. The rear frame assembly 104 can include an operator cab 114 in which an operator controls the vehicle 100.
The work implement can be controlled hydraulically using one or more conventional spool valves. A conventional spool valve can meter inlet and exhaust fluid flow through a housing to control the raising and lowering of the work implement. Fluid metering between a hydraulic cylinder, pump, and tank reservoir controls the work implement during operation. The cylinder can be controlled by the speed at which fluid flows through the inlet and exhaust passages of a valve housing. In a conventional housing, the conventional spool valve is either open or closed to fluid flow and thus there is no independent metering of flow.
In the embodiment of FIG. 1, for example, the loader bucket 106 can be lowered from a raised position (not shown) to a lowered position (as shown). To do so, the loader bucket 106 is lowered in the direction of gravity creating an over-running load condition. To prevent the loader bucket 106 from uncontrollably collapsing to the lowered position, fluid flows through a conventional spool valve. In particular, the conventional spool valve is controlled to an open position to form a restriction in the hydraulic system, e.g., exhaust flow from the hydraulic cylinder to the tank reservoir. However, in the lowered position, the conventional spool valve continues to remain in the open position to allow fluid to flow into the hydraulic cylinder, but the restriction remains as flow is exhausted. In other words, in a conventional spool valve arrangement, there is always a restriction because the inlet and exhaust cannot be independently controlled. As a result, there is unnecessary waste of energy through the conventional spool valve. To overcome this disadvantage, conventional hydraulic systems require multiple spool valves and a redesigned valve housing to incorporate the additional spool valves.
A need therefore exists to provide an improved way of metering flow through a hydraulic system to overcome the disadvantages of conventional spool valves without redesigning the valve housing and incorporating additional spool valves.